Fuel cells transform a fuel and an oxidizing agent at locally separated electrodes into current, heat and water. Hydrogen or a hydrogen-rich gas can be used as fuel and oxygen or air can be used as oxidizing agent. The process of energy transformation in fuel cells has an especially high efficiency. For this reason, fuel cells in combination with electric motors increasingly gain importance as an alternative to customary combustion engines.
Polymer electrolyte membrane fuel cells (PEM fuel cells) are especially well suitable for the use as energy transformer in vehicles as well as in stationary applications because of its compact assembly, power density and high efficiency. A PEM fuel cell can be operated with various fuels or combustion gases. For example, pure hydrogen or hydrogen containing reformate gases can be used in a PEM fuel cell. Liquid methanol can be used in a direct methanol fuel cell. These fuels are transformed oxidatively at the anode by releasing cationic species, in most cases protons, and electrons. As an oxidizing agent, pure oxygen or air is used at the cathode.
A membrane electrode assembly (MEA) has a multi-layer structure. A polymer electrolyte layer capable of conducting ions forms the central layer. Both of the surfaces of the membrane being opposite to each other are in contact with one catalytic layer on which are disposed hydrophobic impregnated gas diffusion layers (so-called GDLs or “backings”). The assembly consisting of a catalytic layer and a gas distribution substrate is called an electrode. One electrode is the cathode and the other electrode is the anode of the membrane electrode assembly.
The catalytic layers comprise a mixture of an ionomer with an electro-catalyst and eventually a bonding material like, e.g., PTFE. Precious metal blacks (small particles of platinum or its alloys) or supported catalysts of finely divided carbon particles, like carbon black, to which the precious metal is applied in high dispersion, are adequate for catalysts.
The polymer electrolyte membrane is formed of a proton-conducting polymer material and has a thickness of 20 μm to 200 μm. These materials are also called “ionomer”, in short. Preferably, a tetrafluoroethylene-fluorovinylether copolymer with acid functions, especially sulphonic acid, is used. E.I. DuPont distributes such a material under the trademark Nafion®. But other materials, especially fluorine-free ionomer materials, such as sulphonated polyether ketones or sulphonated acryl ketones as well as doped polybenzimidazole can be used too.
The gas diffusion layers (GDLs) are made of high porous, electrically conducting carbon fiber substrates, such as, e.g., carbon fiber paper, carbon fiber fleece or a carbon fiber fabric with a thickness of 100 μm to 400 μm and a porosity of more than 50% up to 95%. The average pore diameter of the gas diffusion layer is in the range of 30 μm to 50 μm. In order to avoid the condensation of water in the pores of the gas diffusion layers, the carbon fiber substrates are impregnated by a suspension of a hydrophobic polymer, preferably polytetrafluoroethylene (PTFE), and finally calcined at a temperature in the range of the melting point of the polymer.
For the production of porous catalytic layers a paste, also called a catalyst ink, from ion conducting polymers dispersed in a solvent and an electrocatalyst is prepared, applied with known application techniques to the carbon fiber substrate and then the solvent is evaporated at moderate temperatures. The ion-conducting polymer in the catalyst ink is normally the same as the one of which the membrane is made. The porous structure of the electrode layers guarantees an optimal three-phase-contact between the ion conducting ionomer, the catalyst and the gaseous reactants. Thus, an easy exchange of protons between the polymer electrolyte membrane and the active centers of the catalyst is achieved.
Often it is necessary to repeat the coating several times, in order to achieve a certain load of precious metals in the catalyst layers.
There are a lot of different methods for the production of membrane electrode assemblies, but only a few are sufficient for industrial production, which means for a continuous production in high numbers at high economic costs.
It is known in the art that the polymer electrolyte membrane can be coated by a transfer method (decal method), wherein membranes in ion exchanged form (e.g., Na+-form) are used. Thin catalyst layers are produced having a layer thickness of less than 10 μm. The method comprises many processing steps, is cumbersome and cost intensive and thus only adequate for small series.
It is also known in the art that there is a method for the application of electrode layers on a strip-shaped polymer electrolyte membrane; the method is used in the production of membrane electrode assemblies for PEM fuel cells. On the front and back surface of the polymer electrolyte membrane having a water content of 2 to 20 wt.-% a predetermined pattern of the catalyst layers is continuously printed using an ink containing an electrocatalyst. The printed catalyst layers are dried immediately after printing at elevated temperatures. In the following, the required gas diffusion layers are applied to the free surfaces of the catalyst layers by adhering, pressing or laminating. This method allows a continuous production of MEAs in industrial applications.
If the coating has to be repeated in order to achieve a desired concentration of precious metals, problems occur when using catalyst inks containing mostly organic solvents. Due to the absorption of the organic solvents, the membrane swells considerably and thus creates problems in keeping accurate dimensional stability. The membrane distorts and can fold.
The prior art describes a method for the continuous production of a composite comprising an electrode material, a catalyst material and a polymer electrolyte membrane wherein a dry catalytic powder containing the electrode material, the catalyst material and the material of the solid electrolyte is used to form a catalyst layer on a carrier. This catalyst layer is heated on the side not facing the carrier for softening the solid electrolyte material and is rolled under pressure onto the polymer electrolyte membrane. Disadvantages of this method are dust production and high production costs. If there are coarse powder particles in the catalytic material, the membrane can additionally be perforated (development of pinholes) when the powder is rolled on.
The prior art also describes a continuous method for the coating of a polymer electrolyte membrane with catalytic components, wherein a strip-shaped polymer membrane is pulled through a bath of platinum salt solution. The adhering salt is then reduced to precious metal in a gas stream or a further bath. With this method, the polymer electrolyte membrane is coated on both surfaces. The solution of platinum salt can penetrate the membrane that leads to a deposition of precious metal in the interior of the membrane during the reduction. Additionally, the membrane can be damaged or polluted, due to the very acidic metal salt solution and the reduction bath.
Furthermore, the prior art describes a method for the production of a membrane electrode assembly wherein the bonding of the polymer electrolyte membrane with the catalyst layers and the gas diffusion layers is achieved by a rolling process. This method is cumbersome, cost intensive and is not quite adequate for mass production.
Alternatively it is also known to apply the catalyst layers on the gas diffusion layers (GDLs). Thus, e.g., commercial electrodes of the company E-TEK are available which comprise a coating of a platinum supported catalyst on carbon black on a conventional gas diffusion layer. For the construction of a fuel cell, these electrodes are applied on both sides of a polymer electrolyte membrane. According to the prior art, the electrical contact of the electrodes to the membrane can be improved by impregnating the catalyst layers with, e.g., a solution of an ionomer. The impregnated electrodes are dried before using them in the production of a fuel cell. In the following, the electrodes are pressed or laminated with a polymer electrolyte membrane to form a membrane electrode assembly. During this production step high temperatures (>120° C.) and high pressures (up to 60 bar) are used, thus the membrane can be damaged or perforated by the gas diffusion layer (e.g., by sharp parts of the carbon fiber fleece or carbon fiber paper). Another disadvantage of the lamination process is that the porous gas diffusion layers, too, can be damaged or compressed irreversibly during the exertion of pressure. This can impair the electric properties and life of the completed membrane electrode assembly.
Based on the forgoing, there is a need in the art for a method for the production of a membrane electrode assembly that guarantees an optimal bonding of the catalyst layers with the polymer membrane. Furthermore, a method should be found which avoids the disadvantages of the multiple direct coating of the membrane (especially the accuracy problems when printed repeatedly) and the disadvantages of the pressing and lamination methods (especially the damaging of the membrane and/or the gas diffusion layer due to high pressures and high temperatures).